MINOR: pattern: add a counter of added/freed patterns

MINOR: pattern: add a counter of added/freed patterns

Patterns are allocated when loading maps/acls from a file or dynamically
via the CLI, and are released only from the CLI (e.g. "clear map xxx").
These ones do not use pools and are much harder to monitor, e.g. in case
a script adds many and forgets to clear them, etc.

Let's add a new pair of metrics "PatternsAdded" and "PatternsFreed" that
will report the number of added and freed patterns respectively. This
can allow to simply graph both. The difference between the two normally
represents the number of allocated patterns. If Added grows without
Freed following, it can indicate a faulty script that doesn't perform
the needed cleanup. The metrics are also made available to Prometheus
as patterns_added_total and patterns_freed_total respectively.